This invention relates to a tap control circuit for controlling controllable tap gains which is used in an adaptive filter.
An adaptive filter is supplied with a received signal through a transmission path to filter the received signal into a filtered signal in accordance with first through N-th controllable tap gains, where N represents a positive integer which is not less than one. The adaptive filter may be one of an echo canceller, a howling canceller, and an adaptive equalizer.
A conventional tap control circuit comprises a producing section for producing the first through the N-th controllable tap gains dependent upon the filtered signal and a control parameter which is representative of a varying rate of the first through the N-th controllable tap gains.
The received signal may be received through an input path at the adaptive filter. When taking a delaying time of the input path into consideration, it is necessary to make the positive number N very large in order to filter the received signal into a filtered signal as desired. Therefore, it is practically difficult to filter the received signal into the filtered signal because the conventional tap control circuit must control the first through the N-th controllable tap gains.
In order to practically filter the received signal into the filtered signal, an improved tap control circuit is disclosed in an article contributed by Shinichi Kawamura and Mitsutoshi Hatori to ICASSP of the IEEE (held 1986 in Tokyo), pages 2979 to 2982, under the title of "A TAP SELECTION ALGORITHM FOR ADAPTIVE FILTERS". The improved tap control circuit further comprises a controlling section for controlling the producing section to select selected ones of the first through the N-th controllable tap gains as first through K-th selected tap gains in accordance with the first through the N-th controllable tap gains, where K represents a positive integer which is less than the positive number N and is not less than one. This means that the adaptive filter filters the received signal into the filtered signal in accordance with the first through the K-th selected tap gains.
However, the control parameter is not varied in accordance with the first through the N-th controllable tap gains in the improved tap control circuit on selecting the first through the K-th selected tap gains from the first through the N-th controllable tap gains. This makes it difficult to rapidly select the first through the K-th selected tap gains from the first through the N-th controllable tap gains. Therefore, it is difficult to filter the received signal into the filtered signal in the adaptive filter at a rapid speed.